In an internal combustion engine using the Otto cycle, an inlet air/fuel mixture is compressed adiabatically before ignition. Compression of the inlet air/fuel mixture causes a corresponding increase in temperature resulting from the energy input during compression. Increasing the compression ratio in the cylinder is an approach that has been followed to increase the efficiency of such engines. However, by increasing the compression ratio in the cylinder, the temperature of the air/fuel mixture in the cylinder increases, and can lead to engine knock or in some circumstances pre-ignition, having damaging effects on the engine.
In such engines, sub-optimal charge conditions in the cylinder can result in sub-optimal combustion, which can lead to, for example, incomplete combustion resulting in slip, poor efficiency or knock that in extreme cases can have deleterious effects on the engine.